1. Field of the Invention
The present invention relates to a method for manufacturing a magnetic recording medium, particularly to a method for manufacturing a high-density magnetic recording medium showing little wear of a head and excellent in running durability.
2. Description of the Related Art
In the magnetic recording in recent years, the wavelength of recording tends to be shorter with the increase of density. The requirement is then increasing for a magnetic recording medium to reduce a spacing loss by providing surface properties of a magnetic layer with higher smoothness. One of the methods to provide surface properties of a magnetic layer with smoothness is to strengthen the dispersion of the magnetic liquid, which is applied on a support to form the magnetic layer in manufacture of the magnetic recording medium. In common media dispersion, the dispersion has been strengthened by using as media beads the media having high specific gravity such as zirconia for dispersing long time, by increasing the loading ratio of the media beads or by increasing peripheral velocity.
In the case where the surface properties of the magnetic layer are provided with smoothness by these methods, finely divided particles of abrasives are unevenly distributed on the very surface layer of the magnetic layer, causing problems such as the reduction of output due to the reduction of the degree of filling of the magnetic material in the very surface layer and the increase of noise in the magnetic medium Moreover, die reduction of the height of projected abrasives on the surface of the magnetic layer causes a problem of reducing running durability. Furthermore, the above-described problems will become more obvious, if the abrasives to be used are made finely divided particles for providing smoothness to the surface properties of the magnetic layer.
Japanese Patent Application Publication No. 07-014159 discloses one of the methods to solve this problem in which the abrasives and carbon black in a magnetic liquid are formulated to prepare a paste, separately from the magnetic coating including a ferromagnetic powder and a binder, and they are separately subjected to dispersion treatment, before mixing. In this method, a magnetic coating, an abrasive paste liquid and a carbon black paste liquid are each separately dispersed into optimum dispersion states, and they are mixed and prepared to produce a coating liquid. This method has substantially solved the problem of the degree of filling in the above-described magnetic layer and the problem of the reduction of durability due to the reduction of the projected height on the surface of a magnetic layer.
However, the method for producing the magnetic coating by separate dispersion and mixing has newly caused the problem of the increase of wear of a magnetic head to be in contact with the magnetic recording medium due to the aggregation of the magnetic material and the problem of the reduction of durability due to the aggregation of carbon black Thus, it is difficult to manufacture a magnetic recording medium that has a smoother magnetic layer surface, high output and excellent running durability and shows little wear of a head. It is desired that these properties are compatible with each other.